The present invention relates generally to method of processing downhole property measurements. More particularly, the present invention relates to a method for eliminating the effects of lateral tool motion on formation resistivity measurements and other downhole measurements.
Modem petroleum drilling and production operations demand a great quantity of information relating to the parameters and conditions downhole. Such information typically includes the location and orientation of the well bore and drilling assembly, earth formation properties, and drilling environment parameters downhole. The collection of information relating to formation properties and conditions downhole is commonly referred to as xe2x80x9cloggingxe2x80x9d, and can be performed by several methods.
In conventional wireline logging, a probe or xe2x80x9csondexe2x80x9d having various sensors is lowered into the borehole after some or all of the well has been drilled. The sonde is typically constructed as a hermetically sealed, steel cylinder for housing the sensors, and is typically suspended from the end of a long cable or xe2x80x9cwirelinexe2x80x9d. The wireline mechanically suspends the sonde and also provides electrical conductors between the sensors and electrical equipment located at the surface of the well. Normally, the cable carries power and control signals to the sonde, and carries information signals from the sonde to the surface. In accordance with conventional techniques, various parameters of the earth""s formations adjacent the borehole are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole.
An alternative to wireline logging entails the collection of data during the drilling process itself. Designs for measuring conditions downhole along with the movement and location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as xe2x80x9cmeasurement-while drillingxe2x80x9d techniques, or xe2x80x9cMWDxe2x80x9d. Similar techniques, concentrating more on the measurement of formation parameters, have commonly been referred to as xe2x80x9clogging while drillingxe2x80x9d techniques, or xe2x80x9cLWDxe2x80x9d. While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term LWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
The sensors used in a wireline sonde or a bottom hole assembly may include a source device for transmitting energy into the formation, and one or more receivers for detecting the energy reflected from the formation. Various sensing tools have been used to determine particular characteristics of the formation, including nuclear sensors, acoustic sensors, and electrical sensors.
For an underground formation to contain petroleum, and for the formation to permit the petroleum to flow through it, the rock comprising the formation must have certain well known physical characteristics. For example, one characteristic is that the rock in the formation have space to store petroleum. If the rock in a formation has openings, voids, and spaces in which oil and gas may be stored, it is characterized as xe2x80x9cporousxe2x80x9d. Thus, by determining if the rock is porous, one skilled in the art can determine whether or not the formation has the requisite physical properties to store and yield petroleum. Various well known sensors may be used to measure formation porosity.
Once the porosity has been determined, other sensors are used to identify the fluids held by the porous rock formations. One property that may be used to distinguish between liquid petroleum and brine in a formation is the formation resistivity. Porous formations having a low resistivity are likely to contain brine, whereas formations that contain petroleum are likely to have a high resistivity. In a type of formation called xe2x80x9cshaley-sand,xe2x80x9d for example, the shale bed can have a resistivity of about 1 ohm-meter. A bed of oil-saturated sandstone, on the other hand, is likely to have a higher resistivity of about 10 ohm-meters or more. The sudden change in resistivity at the boundary between beds of shale and sandstone can be used to locate these boundaries. Various tools well known to those of skill in the art may be used to acquire the resistivity measurements. Examples of suitable tools include galvanic tools, induction tools, and other tools that measure resistivity.
Induction tools typically include a sonde having a transmitter coil and one or more receiver coils at locations axially spaced from the transmitter coil. The basic element in all multi-coil induction tools is the two-coil sonde. The two-coil sonde consists of a single transmitter coil and a single receiver coil wrapped around an insulating mandrel. The transmitter coil is driven by an oscillating current at a frequency of a between few tens of kilohertz and a few megahertz. The resulting magnetic field induces eddy currents in the formation which are coaxial with the tool. These eddy currents cause a secondary magnetic field which in turn induces a voltage in the receiver coil. This induced voltage is then amplified, and the component of the voltage that is in-phase with the transmitter current is measured and multiplied by a tool constant to yield an apparent conductivity signal. This apparent conductivity is then recorded at the surface as a function of the depth of the tool.
Induction tools (as well as galvanic resistivity tools) commonly measure the resistivity in a concentric volume around the tool. Experts generally speak of this volume as having an average radius which is called the xe2x80x9cdepth of investigationxe2x80x9d or xe2x80x9cradius of investigation.xe2x80x9d In addition to the formation resistivity, the response of induction tools (as well as other resistivity measurement tools) is a function of several factors including the borehole geometry, the borehole fluids, and the position of the tool in the borehole. The response of the tool is therefore a multivariable function which can be calculated using a mathematical model, or alternatively, which can be empirically measured during a calibration process. A competent computer programmer can normally write a computer routine that will calculate the expected tool response after receiving the various factors. However, because the expected tool response is only rarely a closed-form equation which can be solved for the formation resistivity, writing a program that calculates the formation resistivity after receiving the actual tool response and various other factors presents some difficulty. This difficulty is aggravated if the tool response was measured while one or more of the factors is changing.
The problems described above are in large part solved by a method of compensating for tool motion. In one embodiment, the method includes passing a logging tool through a borehole, obtaining a series of tool response measurements, and obtaining a series of tool position measurements associated with the tool response measurements. The tool position measurements preferably indicate the distance between the tool axis and the borehole axis. The tool position measurements are preferably examined to determine if the tool was moving while the tool was making measurements. For measurements made while the tool was moving, the measurement time interval is preferably divided into subintervals each having a corresponding tool position. A formation property (such as resistivity) is estimated, and the expected tool response for each tool position is calculated. The expected tool responses are combined to form a model tool response, that is, the tool response that might be expected for a tool in motion. The model tool response is then compared to the measured tool response. The estimate of the formation property is adjusted and the process is repeated until the model tool response is substantially equal to the measured tool response.